1. Field of the Invention
The present invention generally relates to an NC machine tool with a tiltable spindle unit. More specifically, the invention relates to an NC machine tool with a tiltable spindle unit for selectively using a vertical spindle and an inclined spindle in a tool mounting position.
2. Description of the Prior Art
In recent years, NC machine tools capable of shaping optical parts ultra-precisely, such as lenses, have been developed. An ultra-precise NC machine tool of this type can work a concave surface of a lens precisely by using a grindstone or end mill with a flat or spherical end. Conventional NC machine tools for shaping concave surfaces of lenses or the like include vertical spindle type machines, the tool mounting position of which extends in vertical directions, and inclined spindle type machines, the tool mounting position of which is inclined.
FIG. 6a schematically shows a concave surface grinding using a vertical spindle type machine, FIG. 6b shows a concave surface grinding using an inclined spindle type machine.
As shown in FIG. 6a, in a case where a concave surface of a workpiece W with a relatively small diameter is grinded by means of a grindstone 1 attached to a vertical spindle 3, the vertical spindle 3 interferes with the outer circumferential edge portion on the concave surface of the workpiece W, if the radius of curvature on the concave surface of the workpiece W is large and if the spindle 3 having the grindstone 1 is relatively thick with respect to the radius of the grindstone 1. To avoid the interference, the diameter of the spindle 3 must be as small as possible.
However, if the diameter of the workpiece W is 3 mm or less, the rigidity of the spindle 3 attached to the grindstone 1 is reduced, so that a precise working can not be achieved. In a case where a spindle 4 attached the grindstone 1 with a larger diameter having a high rigidity is tilted as shown in FIG. 6b, it is possible to work the concave surface without interfering with the workpiece W. Thus, conventionally, the vertical spindle and the inclined spindle are selectively used for working the concave surface of a small work having a diameter of a few millimeters while insuring the rigidity of the spindles.
FIGS. 7 and 8 show a conventional NC machine tool capable of being used both as a vertical spindle type machine and an inclined spindle type machine in accordance with a work to be grinded or milled. In FIG. 7, a spindle is set to a vertical orientation. In FIG. 7, reference number 10 denotes a column with slide, 12 denotes a spindle unit for vertical oriented spindle, and 14 denotes a rotary unit for rotating a workpiece mounted thereon. The column with slide 10 is designed to be fed perpendicular to the plane of the figure by a control axis extending in directions of X-axis. The spindle unit 12 is detachably mounted on a slide 17 which is vertically movable along a slideway of the column with slide 10 via a vertical spindle mounting base 16. The control axis for feeding the spindle 12 is Y-axis. A vacuum chuck 18 is mounted on the end portion of the rotating shaft of the rotary unit 14, and the workpiece W is held on the vacuum chuck 18 via a fixture 19 by vacuuming air. The rotary unit 14 is installed on a table 22. The table 22 is designed to be fed in directions of Z-axis as control axes, perpendicular to Y-axis and X-axis. The spindle unit 12 shown in FIG. 7 includes a drive unit for a spindle 20. On the top end of the spindle 20, a tool, such as a grindstone or an end mill, is mounted.
On the other hand, FIG. 8 shows a operating manner in which a spindle is changed to an inclined spindle type. The spindle can be changed to an inclined spindle type by replacing the spindle unit 12 for vertical spindle shown in FIG. 7 with a spindle unit 24 for inclined spindle. In this case, the spindle unit 24 for inclined spindle is detachably mounted on the slide 17 via an inclined spindle mounting base 25.
Such a conventional NC machine tool can work precisely the concave surface of the workpiece W by making replacement selectively between the spindle unit 12 for vertical spindle shaft with the spindle unit 24 for inclined spindle without changing other mechanical parts, such as the column with slide 10 and the rotary unit 14.
In an NC machine tool of this type, it is required to measure a tool length, a tool diameter and a working point of the tool for preparations of the machining. Because the tool length, the tool diameter and so forth are data indispensable for grasping the working point of the tool precisely to conduct the position-control the spindle. In the case of the vertical spindle, it is easy to measure the tool length, the tool diameter and so forth, whereas in the case of the inclined spindle, it is very difficult to precisely measure the tool diameter and the working point the tool attached to the spindle 20 since the tool is inclined on the mounting position.
For that reason, in the case of the inclined shaft, it is the conventional practice that a tool wherein its diameter and so forth have been measured is mounted, and the measured values are used for preparing NC data, so that the alignment of a work piece with the tool is carried out on the basis of experience and practical sense after trial-and-error.
In the case of the inclined spindle, there is a problem in that it is very difficult to precisely carry out truing in addition to the problem on tool measurement, since the grindstone mounting position is inclined. Particularly, if the grindstone is eccentric, it is very difficult to carry out truing for correcting eccentricity.
Moreover, in FIGS. 7 and 8, it is assumed that the distance between the end face of the vacuum chuck 18 and the slide 17 of the column with slide 10 is L, the distance between the end face of the vacuum chuck 18 and the center of the tool is L1, and the distance between the center of the tool and the slide 17 is L2. In the case of the inclined spindle, the distance L1 between the end face of the vacuum chuck 18 and the center of the tool is far shorter than that in the case of the vertical spindle. Therefore, in order to insure a constant stroke in Z-axis in the case of the inclined shaft, the whole machine must be large, or the column with slide 10 must be replaced.
It is therefore an object of the present invention to eliminate the aforementioned problems and to provide an NC machine tool with tiltable spindle unit capable of being used both as vertical and inclined spindle type machine tools without restrictions on stroke and capable of easily carrying out tool measurement, alignment of a work piece with a tool, and truing.
In order to accomplish the aforementioned and other objects, according to one aspect of the present invention,
an NC machine tool with a tiltable spindle unit having a plurality of control axes, comprises: a spindle unit having a spindle serving both as a vertical spindle for keeping a tool in a vertical position and an inclined spindle for keeping the tool in a inclined position; a column for installing the spindle unit thereon; a table for supporting a workpiece; a turning shaft for pivotably supporting the spindle unit on a vertical plane, and a tilting means for changing the position of the spindle unit between the vertical position and the inclined position; wherein the turning shaft has a center of rotation which is arranged in the vicinity of a top end of the tool in alignment with a center line of the tool.
The tilting means part may comprises a mounting bracket mounted on a slide which vertically moves along a guide on a side of the column; a spindle unit bracket to which the spindle unit is fixed and which is connected to the turning shaft; a bearing, fixed to the mounting bracket, for horizontally supporting the turning shaft; and a positioning means for selectively positioning the spindle unit at the vertical position and the inclined position.
The NC machine tool with pivot may further comprise a a rotary unit, located on the table, for rotating the workpiece on a center line of the workpiece, said rotary unit having a chuck means capable of holding the workpiece
The NC machine tool may further comprise measuring means for measuring a distance from a center of the workpiece to an end face of the tool in a vertical attitude, and a diameter of the tool to output measured data to a numerical control unit.